Studies were performed to investigate the regulation of carbohydrate and energy metabolism (i.e., control of adenine nucleotides) in human skeletal muscle during exercise and euglycemic hyperinsulinemia and the regulation of lactic acid production during muscle contraction. It was found that lactate production was preceded by or occurred in parallel with increases in mitochondrial nicotinamide adenine dinucleotide reduced (=NADH), suggesting, in contrast to current views that lactate production during submaximal exercise is oxygen-dependent. Glucose 1,6- bisphosphate (GP2), an important regulator of key enzymes of carbohydrate metabolism, has been shown to increase after isometric contraction to fatigue (approximately 50 seconds). To study the regulation of GP2 contents in muscle during contraction, we obtained biopsies prior to, after 20 seconds of contraction, and at fatigue. The major increase in GP2 occurred within the first 20 seconds of exercise, with no significant change there after. Preliminary results suggest that the rapid increase in GP2 is due to activation of GP2 synthase by its substrates G-1-P and G-6-P. The lack of continuous production, while the substrates increase, during the latter part of contraction, may be due to allosteric inhibition (by inorganic phosphate) of the synthase and/or activation of GP-2 phosphatase (by Ca2+ or inosine monophosphate (IMP)). A series of studies were performed to determine the in vivo regulation of AMP deaminase (deaminates AMP to IMP and ammonia), the activity of the purine nucleotide cycle (responsible for reaminating IMP back to AMP (and thereby ATP (via myokinase)). Last, it has been suggested that during euglycemic hyperinsulinemia, free glucose accumulates in the muscle of insulin.